Climate link with killer cyclones spurs fierce scientific debate
Pastor Dale Morgan
Climate link with killer cyclones spurs fierce scientific debate*
Published on Wednesday, May 7, 2008
By Richard Ingham and Anne Chaon
PARIS, France (AFP): In 2005, Hurricane Katrina laid waste to parts of
the US Gulf Coast.
Last year, the Arabian peninsula was hit by a super-cyclone, Gonu.
Now, an unusual early-season storm, Nargis, has slammed into Myanmar,
brutally changing gear from a Category One to a Category Four cyclone
just before it made landfall.
Are these events -- massively costly in lives and treasure -- all linked?
Could they be part of an alarming trend of weird, more powerful storms
stoked by global warming?
That's a question that causes fierce jousting among climate scientists.
Experts agree that a single weather event cannot be pinned to climate
change, which is part of a long-term pattern spanning decades or centuries.
"It's impossible to say," Adam Lea of the Benfield UCL Hazard Research
Centre at University College London told AFP.
"It's only in the long term that you get the perspective that lets you
say whether an extreme event is part of a wider trend," said French
researcher Herve Le Treut, who contributed to last year's landmark
report by the UN's Intergovernmental Panel on Climate Change (IPCC).
But that's where the scientific consensus ends.
Some experts argue the evidence is already hard enough to identify a
probable trend: storms are becoming more powerful as global warming
heats up the oceans.
One of the most respected voices in the field is that of Massachusetts
Institute of Technology (MIT) meteorology professor Kerry Emanuel, who
calculates that the power of tropical cyclones has roughly doubled since
the 1950s.
The massive increase has especially occurred over the last three
decades, mirroring a rise in man-made global warming, he notes. And the
trend stepped up a couple of gears from the mid-1990s, when global mean
temperatures began to scale ever-higher annual peaks.
Others, though, say these judgements are premature.
They argue that we still need long-term historical data -- in which big
weather oscillations and cycles in hurricane activity are filtered out
-- in order to get a clear picture.
Far more is known about storm activity in the Atlantic, for instance,
than in the Indian Ocean, and the Atlantic records themselves go back
only 30 years or so, to the advent of satellite monitoring, Le Treut noted.
A study published last year by Johan Nyberg of Sweden's Geological
Survey used Caribbean corals, whose growth is affected by temperature
and nutrients stirred up by storms, to get a view spanning two and a
half centuries.
Nyberg concluded that 1971-94 was abnormally calm for hurricane activity
and that the big increase in storm numbers since 1995 was "not unusual"
when compared to the longer record.
Tropical storms are called hurricanes when they occur in the Atlantic,
typhoons when they happen in the Pacific and cyclones when they brew in
the Indian Ocean.
The basic cause is the same -- heat and moisture provided by seas warmed
to at least 26 or 27 degrees Celsius (78.8-80.6 degrees Fahrenheit).
But another factor is vertical wind shear, or the angle of prevailing winds.
This determine whether the nascent storm develops into the notorious
wheeling "eye" of a cyclone or is torn into harmless shreds.
It's still unclear what impact global warming will have on vertical wind
shear, say some experts. A theoretical combination of lower wind shear
and warmer seas could result in storms that last longer, are more
vicious and more frequent, too.
The IPCC's 2007 report said tropical cyclones were "likely" to become
more intense, packing higher winds and rain, by 2100. But it also
highlighted the fact that human settlement in vulnerable areas increased
the toll from when the storms strike.